1.Field of the Invention
The invention relates to compositions which comprise organic compounds having aromatic radicals attached directly to the silicon atom and to their use as defoamers.
2.Description of the Related Art
In many liquid systems which include surface-active compounds as desired or as unwanted constituents, especially in aqueous systems, it is possible for problems to occur as a result of foaming if these systems are contacted more or less intensively with gaseous substances, such as during the gassing of wastewaters, during the intensive stirring of liquids, during distillation, washing or coloring operations or during dispensing processes, for example.
This foam can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamers have proven particularly appropriate.
Siloxane-based defoamers are prepared in accordance with DE-B 15 19 987, for example, by heating hydrophilic silica in polydimethylsiloxanes. Using basic catalysts allows the effectiveness of such defoamers to be improved, as disclosed in DE-A 17 69 940, for instance. An alternative is to disperse hydrophobicized silica in a polydimethylsiloxane, in accordance for example with DE-A 29 25 722. Nevertheless, the effectiveness of the resulting defoamers is still in need of improvement. Thus U.S. Pat. No. 4,145,308, for example, describes a defoamer preparation which in addition to a polydiorganosiloxane and silica further comprises a copolymer made up of (CH3)3SiO1/2 and SiO2 units. Copolymers made up of (CH3)3SiO1/2 and SiO2 units are also said to be advantageous in combination with siloxanes which carry terminal long alkyl groups, as described for instance in EP-A 301 531. The use of partly crosslinked polydimethylsiloxanes which are in some cases already rubberlike is said to contribute to increasing the defoamer effect. On this point reference may be made, for example, to U.S. Pat. No. 2,632,736, EP-A 273 448 and EP-A 434 060. These products, though, are generally of very high viscosity and are difficult to handle or to process further.
Generally, use is made of polysiloxanes having methyl groups, such as polydimethylsiloxanes. Although polymers with a range of other aliphatic or aromatic hydrocarbon groups on the silicon are known and are also proposed in numerous patents for the preparation of defoamers, there is little indication that by selecting the substituents on the silicon it is possible to achieve a substantial improvement in the defoaming effect. Frequently the aim of introducing long alkyl groups or polyether substituents is to improve the compatibility with mineral oils that may be present in defoamer compositions, or to prevent silicone defects in coatings, for example. Thus EP-A 121 210 recommends the use of polysiloxanes which carry alkyl groups having 6-30 carbon atoms, so that the fraction of carbon in the form of the CH2 group is 30%-70%, in combination with mineral oil. In the examples, mention is made in particular of polysiloxanes having octadecyl groups. JP-A 60173068 recommends siloxanes having octyl groups and polyether groups as defoamers in aqueous printing inks. Siloxanes having alkyl groups with more than 30 carbon atoms in combination with amino siloxanes are said by U.S. Pat. No. 4,584,125 to be advantageous for the antifoam effect especially when the fraction of siloxane units carrying these moieties is around 5%.
In strongly foaming, surfactant-rich systems, however, the defoamer formulations prepared in accordance with the prior art do not always have a sufficiently long-lasting effectiveness or else, owing to the high viscosity, because of the degree of branching or crosslinking that is achieved, are difficult to handle.